1. Field of the Invention
This invention relates to a fluid flow system for metering and monitoring flow from multiple liquid sources and more particularly to a plasmapheresis blood flow control system which meters and monitors liquid flow from several sources isolated from handling noise and gives advanced warning when fluid sources are depleted.
2. Discussion of the Prior Art
In fluid flow systems the need arises for controlling sequential or simultaneous influx from multiple fluid sources, especially those of limited capacity. In some cases it is desirable to monitor the total influx from such fluid sources so as to match the amount of fluid withdrawn from the system at another point or in another operation. For instance, plasmapheresis systems are known which require the withdrawal from and subsequent reinfusion of bodily fluids to a living subject in known or fixed amounts. The subject is usually a human or animal, but might also be a cadaver.
In the case of a plasmapheresis system, whole blood is extracted from the subject, plasma is separated from the whole blood, and an extraction product containing a higher concentration of blood cells than the whole blood is reinfused into the subject while the separated plasma is retained and used for desired purposes, treated to alleviate an undesirable condition, or discarded as unfit for further use. To compensate for plasma removed from the patient, a selected volume of replacement fluid is infused into the patient to replace the volume of plasma separated from the whole blood. The use of replacement fluids often has a therapeutic aim. Many undesirable components of blood, including diseased cells, antigens, and the like, are held in suspension therein. During therapeutic plasma exchange, these undesirable components are separated from the blood and removed with the plasma fraction, leaving the blood cleansed and healthier. The replacement fluids used during therapeutic plasma exchange can comprise a variety of fluids, such as saline, thawed plasma, and various therapeutic fluids, depending upon the desires of the treating physician. In such a case it is often desirable to control the amount and rate of each of the replacement fluids administered to the patient.
Determining the exact amount of any given replacement fluid administered to the patient during therapeutic plasma exchange is complicated by the use in modern plasmapheresis machines of disposable plastic tubing harness sets. Although manufactured to specifications as exact as possible, the flow characteristics of each plastic harness set differ somewhat from those of all others.
Therefore, despite all efforts to calibrate the peristaltic pumps with which the harness sets are used to provide sterile, non-invasive therapy, the pumps deliver at slightly different rates dependent upon the individual flow characteristics of the harness set used. The need exists, therefore, for a method of monitoring and controlling with increased precision the amount of each replacement fluid used during therapy.
Yet another problem inherent in monitoring the weight of process fluids comes from inaccuracies introduced due to motion of the fluids being monitored. For instance, the motion of replacement fluids flowing from a weighing bag is sufficient to affect the accuracy of delicate balances used in weighing by mechanical means. The need exists, therefore, for a way to isolate the fluids weighed from the effects of mass transport.
To optimize use of processing equipment and support personnel while minimizing inconvenience and discomfort to the patient, it is also desirable to reinfuse bodily fluids as rapidly and safely as possible. However, replacement fluids, usually saline, are commonly prepackaged in sterile containers of predetermined volume, necessitating that more than one container of replacement fluid be infused into the patient to replace the plasma removed during a typical session of plasmapheresis. If the containers of saline are fed sequentially into the replacement line for reinfusion, an attendant must monitor the flow of replacement fluid, and manually switch flow from one emptying bag to the next full bag before air enters the replacement line from an empty bag. In some cases the attendant must momentarily stop the flow of replacement fluid to switch flow to a new bag.
If air is inadvertently allowed to enter the infusion line from an empty bag, the machine must be stopped so that air can be removed from the flow line before it enters the patient's blood. In some cases, the entire plasmapheresis session must be scrapped and begun anew. Frequently, if a large amount of blood has been withdrawn, the patient cannot continue with a new session until sufficient time has elapsed to recuperate from the effects of the first session. Time and effort are lost while the patient is exposed to an unnecessary health risk.
The problem of replacement fluid sources going dry is compounded when multiple fluids at different rates and from separate sources are infused either sequentially or simultaneously through the single venipuncture needle. For instance, typically the reinfusion mixture contains concentrated red blood cells and sufficient anticoagulant to prevent coagulation of the red cells. At the same time, a replacement fluid, usually saline, is introduced at a rate sufficient to substitute for the plasma removed. Under certain circumstances it may be desirable to add another fluid to the mixture, such as albumin, frozen plasma, a medicament, or the like. The need exists, therefore, for a method and apparatus adapted to prevent multiple reinfusion fluid sources from going dry while monitoring the total amounts and/or relative infusion rates of the multiple reinfusion sources.
The art has long sought apparatus and equipment useful for monitoring the flow of liquid systems. It is known to monitor the flow of liquids into a fluid flow system by use of various devices. For example, in U.S. Pat. No. 4,655,742 and European patent application No. 232,263, optical detectors are used to determine when a container of fluids is full. Automatic weighing can also be effected by means of an electrical load cell that actuates an electronic device to squeeze off a tube and thereby prevent further filling of a container as disclosed in German patent No. DE 3 739 240.
Weight scales are also known for measuring the flow of blood into or out of a container. For instance, German patent No. DE 3 737 304 discloses a weighing pan connected to a pivot such that at a certain weight of blood in a bag resting on the weighing pan, a compression valve is activated to choke the flow of blood into the bag or interrupt it intermittently. In this way, unnecessary load upon the blood donor's circulatory system and heart are avoided. In addition, European Patent Application EP 879O7352, filed Oct. 11, 1987, discloses a blood separation device that holds and weighs at least two separation bags that communicate with the blood bag by means of a tube. Alternatively, as disclosed in Spanish Patent No. 8 801 535, a mechanical balancing system monitoring the difference between a total instantaneous weight and an instantaneous equilibrium force can be used to regulate the flow of substitute fluid in a blood filtration device.
However, none of these devices warns the attendant when the replacement fluid source is going to run dry, provides a means for switching to an auxiliary source without temporarily stopping flow of replacement fluid into the reinfusion mixture, and/or meters the total amount of replacement fluid used from multiple sources. Thus, the need exists for new and better methods and apparatus for monitoring fluid flow systems, especially in plasmapheresis devices used for reinfusing replacement fluids to compensate for plasma removed and not reinfused.